Prime Minister Theresa May was in Brussels on Thursday to plead with EU leaders to change the Brexit divorce deal she negotiated last year, in order to get it through parliament, after they offered little hope they were willing to do so.

The London parliament, which rejected May’s agreement by the biggest majority in modern British history, has voted to renegotiate the deal, replacing a provision that some fear could keep British-ruled Northern Ireland under EU rules indefinitely, according to Reuters.com.

EU leaders have repeatedly said it would be impossible to replace the provision, known as the “backstop” because it is required to ensure no hard border, once a focus for sectarian violence, between Northern Ireland and EU-member Ireland.

Unless parliament approves a deal, Britain is on course to leave the EU on March 29 with no transition arrangement in place, a scenario that many businesses say would be catastrophic for the economy. Other options could include delaying Brexit, holding a new referendum or canceling it altogether.

May will return to parliament next week for a debate on the Brexit negotiations when lawmakers could again try to wrest control of the process from her, but a crunch vote on approving the Brexit deal is likely to come later in the month.

Both May’s Conservative Party and the main opposition Labour Party are formally committed to carrying out Brexit following a 2016 referendum in which voters chose to leave the EU by a margin of 52-48 percent. But both parties are deeply divided internally over how or even whether to do so.

In a letter to May released on Wednesday, Labour leader Jeremy Corbyn set out five conditions for Labour to support a deal. Those include a “permanent and comprehensive” customs union with the bloc, which May has ruled out.

Corbyn also demanded a close alignment with the single market, “unambiguous agreements” on future security arrangements and commitments on UK participation in EU agencies and funding programmes.

She will tell Juncker, Tusk and the European parliament’s Antonio Tajani she wants to work “urgently” with them to secure changes to the deal reached in November.

The main stumbling block to winning British parliamentary approval of the deal is the Northern Irish backstop, an insurance policy that requires some EU rules to operate in the British-ruled province unless another means can be agreed in future to guarantee a land border free from inspections.

Some lawmakers want May to remove the provision entirely, while others say they will accept a way for London to end it unilaterally, or legally-binding assurances that it would not lead to Britain being trapped in the EU’s sphere indefinitely.

But the EU, and particularly Irish Prime Minister Leo Varadkar say they will not re-open the agreement. They argue that the political instability in Britain has only further proved the need for the backstop.